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How To Optimize Your Solar Pool Heater

To optimize the use of a solar pool heater, it’s important to understand the heat losses and gains from swimming pools. Heat losses from a pool occur primarily through evaporation from the surface, convection and infrared radiation.  This post will explore how to optimize your solar pool heater by mitigating some of the heat losses that are common from swimming pools.

Evaporation can account for as much as 70% of heat losses in a pool.  The endothermic process of evaporation of water, cools the remaining water in the pool.  Each gallon of water that evaporates removes approximately 8,300 Btus of heat from the pool.   On the contrary, warming a gallon of replacement water to the temperature of the pool, requires only 8.3 Btus per degree Fahrenheit of temperature increase or between 80 and 200 Btus per gallon for the typical heat gain required. The relative humidity of the ambient air regulates the evaporation losses, where a lower relative humidity in the ambient air leads to higher evaporation losses, and humid ambient air results in lower evaporation. The wind speed affects the convection loss to a high extent. Convection can be defined as the movement caused within a liquid by the inclination of hotter and therefore less dense material to rise, and colder, denser material to sink under the influence of gravity, which subsequently results in transfer of heat.

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Losses from conduction to the ground are typically small and are often ignored in warmer climates. Outdoor pools can lose some heat via infrared irradiation exchange with the sky.  A transparent cover is an effective way to reduce heat loss from evaporation when a pool is not being used. Air pockets in the cover will reduce convection losses. Savings from pool covers depend on how long the pool is covered and on the local conditions which may impact evaporation.   Additional savings are achieved because less makeup water is required including additional chemicals to ensure the proper pool chemistry. It’s important to note however that water on top of cover (e.g. after rain or leakage) will have a cooling effect. This is due to the fact that as the water evaporates and cools the pool water below, if there is no insulating gap.

Heat gains to a pool may come from direct solar radiation or convection when the ambient temperature is higher than the pool temperature. Transparent covers allow direct solar radiation in the pool while reducing the evaporative losses. Short wave radiation enters the water depending on the wavelength of the radiation. Well-functioning pool covers act as selective transmitters. The covers admit the shorter wavelength radiation of the solar radiation and are reflective of the long wavelengths of the infrared radiation. The infrared radiation accounts for the radiation heat loss e.g. to the cold sky or surroundings. Pool covers reduce the heat loss by evaporation during the day and the radiant heat loss during the night. Researchers have confirmed that the heat losses are dominated by evaporation from the surface of the pool and a cover over the pool at night time significantly reduces the heat losses.

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In conclusion, pool covers block evaporation as well as radiant losses and can provide some insulation from convective losses.   Solar collectors with an area of 50% of the pool surface can work well for heating the pool.  A case study showed that unglazed solar collectors with an area of 62% of the pool surface kept the pool at the required temperature level of 80-85 °C during the colder months.  Implementing these strategies for increasing the pool temperature resulted in an almost doubling the days of the swimming season.  Also, sheltering off wind can significantly reduce heat losses.

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